Microdisplays are miniaturized displays, typically with a screen size of less than 1.5″ in diagonal. Microdisplays are commonly used in data projectors, head mounted displays, and in the traditional viewfinders of digital cameras. They can be implemented within compact projectors, in viewfinders of handheld Internet appliances and in mobile phones for Web surfing and videoconferences, as full computer screens can be viewed in these devices.
Most microdisplays use a light-valve made of a silicon chip as the substrate material. The chip also houses the addressing electronics (at least an active matrix with integrated drivers), usually implemented in standard CMOS technology which allows very reliable and stable circuits, as well as very small pixel pitches (down to 10 μm or even somewhat smaller), as well as high display resolutions.
In the art, there are known reflective and transmissive light valves. The reflective light valves bounce light off the displayed image into the viewer's lens or the projection lens. The transmissive light valves are similar to backlit, portable computer screens using LCD (Liquid Crystal Display) and EL (electro-lumination) technologies. Common reflective light valves are based on Liquid Crystal On Silicon (LCOS) and tilted micro-mirrors (DMD). Common transmissive light valves are based on Active-Matrix Liquid Crystal Displays (AMLCD).
Projectors that use transmissive microdisplays as mentioned above typically comprise an optical path that includes a light source and a Spatial Light Modulator (SLM), in which a beam shaping optic component, as well as a polarizing component are disposed between them. Another polarizing component and a magnifying optic component are generally disposed between the SLM and the projection surface. The SLM is coupled to a video processing driver to produce the image modulation of the light according to an input signal.
Common optical difficulties in the design of known projectors based on a microdisplay are as follows: low energy efficiency; low brightness and non-uniformity of the output image due to the source non-uniform intensity distribution (i.e. Gaussian distribution over the SLM surface) and intensity losses; low focus depth of the output image. In laser based projectors, the “speckle” phenomena of a Laser source according to which a granular pattern of light pervades the image, is also considered as a technical difficulty. Other common difficulties directly related to the optical difficulties and to the hardware implementation are: size, weight, optical complexity, power consumption and the mobility of the overall projecting device.
Different methods and devices addressed to overcome one or more of the above-mentioned difficulties are disclosed by the following.
U.S. Pat. No. 5,971,545 discloses a compact and energy efficient projection display utilizing a reflective light valve. The output beams of the light sources are received by at least one spatial light modulator. The modulated output beams are collimated and combined. A projection lens receives the collimated and combined output beams and directs them towards a projection screen. Energy efficiency is achieved by using sequentially strobed RGB light sources instead of a white light source.
U.S. Pat. No. 5,777,789 discloses an optical system for high-resolution projection display, consisting of reflection birefringent (double refractive) light valves. The LCD projector comprises a polarizing beam splitter, color image combining prisms, illumination system, projection lens, filters for color and contrast control, and a screen. The illumination system includes a light source such as a metal-halide arc lamp, an ultraviolet and infrared filter or filters positioned in the optical path from the light source for filtering out the infrared and ultraviolet light emitted from the light source, a light tunnel for providing uniform light intensity, and a relay lens system for magnifying the illumination system output plane and imaging said plane onto the liquid crystal light valves.
U.S. Pat. No. 5,975,703 discloses an image projection device having an SLM and a polarized source system. The optical system uses polarized light manipulated by at least one of a conicoid, or plane optical elements to affect a folded mirror system to project an image onto a screen by utilizing input light components of more than one state of polarization, thus reducing intensity losses over the optical system due to polarization filtering. The system supplies light components of substantially orthogonal polarizations for separate areas of the SLM to be output onto a projection screen.
U.S. Pat. No. 5,563,710 discloses a confocal laser imaging system including a laser source, a beam splitter, focusing elements, and a photosensitive detector. The laser source projects a laser beam along a first optical path at an object to be imaged, and modulates the intensity of the projected laser beam in response to light reflected from the object. A beam splitter directs a portion of the projected laser beam onto a photodetector, which monitors the intensity of the laser output. The laser source can be an electrically scannable array, with a lens or objective assembly for focusing light generated by the array onto the object of interest. As the array is energized, its laser beams scan over the object, and light reflected at each point is returned by the lens to the element of the array from which it originated. A single photosensitive detector element can generate an intensity-representative signal for all lasers of the array. The intensity-representative signal from the photosensitive detector can be processed to provide an image of the object of interest.
U.S. Pat. No. 6,183,092 discloses a laser projector which includes a laser apparatus and a reflective liquid-crystal light valve capable of speckle suppression through beam-path displacement: by deflecting the beam during projection, thereby avoiding both absorption and diffusion of the beam while preserving pseudocollimation (noncrossing rays). The latter, in turn, is important to infinite sharpness. Path displacement is achieved by scanning the beam on the light valves which also provides several improvements in energy efficiency, brightness, contrast, beam uniformity (by suppressing both laser-mode ripple and artifacts), and convenient beam-turning to transfer the beam between apparatus tiers. The deflection effect is performed by a mirror mounted on a galvanometer or motor for rotary oscillation; images are written incrementally on successive portions of the light valve control stage (either optical or electronic) while the laser “reading beam” is synchronized on the output stage. The beam is shaped, with very little energy loss to masking, into a shallow cross-section which is shifted on the viewing screen as well as the light valves. Beam-splitter/analyzer cubes are preferred over polarizing sheets. Spatial modulation provided by a light valve and maintained by pseudocollimation enables imaging on irregular projection media.
U.S. Pat. No. 5,517,263 discloses a compact size projection system which includes a bright light source of polarized light, and a spatial light modulator, having an alignment layer, to modulate the polarized projection light, wherein the bright polarized light source is aligned with the alignment layer to permit the polarized light to pass therethrough without the need for unwanted light blocking polarizers. The use of a polarized laser source together with its proper alignment with the light valve, enables substantially all of the laser light beams to be utilized by the SLM to form the projected image.
U.S. Pat. No. 5,704,700 discloses a laser illuminated and SLM-based projection system that includes a microlaser array coupled with a beam shaper to produce a bright (i.e. having a uniform intensity distribution) projection light beam to be impinged over the SLM. The beam shaper includes a binary phase plate, a microlens array arrangement or a diffuser arrangement to modify the shape and intensity profile of the projection light beam. The laser light illuminating the light valve thus has a uniform intensity distribution for projecting an extremely bright image, and is confined substantially to the pixel portion of the light valve.